1. Field of the Invention
The present invention generally relates to wireless communications. More particularly, the invention relates to medium access control (MAC) frames and mechanisms enabling smart antenna use, improving channel utilization, and increasing communications throughput.
2. Background Information
Initially, computers were most typically used in a standalone manner. It is now commonplace for computers and other types of electronic devices to communicate with each other over networks. The ability for computers to communicate with one another has lead to the creation of small networks comprising two or three computers to vast networks comprising hundreds or even thousands of computers. Networks can be set up to provide a wide assortment of capabilities. For example, networked computers can be established to permit each computer to share a centralized mass storage device or printer. Further, networks enable electronic mail and numerous other types of services. Networks have been established in a wired configuration in which each entity on the network has a direct physical electrical connection to the network. More recently, advances in wireless technology has made it possible for network devices to communicate with others via radio frequency (RF) or other types of wireless media.
To implement a wireless network, each device (computer, access point, etc.) includes one or more antennas through which data is transmitted or received. One type of antenna configuration is referred to as single input, single output (SISO) and is depicted conceptually in FIG. 1. Two network stations 10 and 12 are shown in communication with each other. The stations could be computers, access points, and the like. In a SISO configuration, each station 10 and 12 includes a single antenna 14 and 16, respectively. Data is communicated between the stations 10, 12 in an exchange sequence via the single wireless link 18.
An exemplary exchange sequence is illustrated in FIG. 2. One of the stations 10, 12 sends a data frame 20 to the other station which responds with an acknowledgment frame 22. The data frame may include a preamble 24, a header 26 and a data payload 28. Similarly, the acknowledgment frame 22 includes a preamble 30, a header 32 and a data payload 34. The data frame conveys data to the receiving station and the acknowledgment frame lets the sending station know that the data frame was correctly received. If the data frame was not correctly received (e.g., due to noise or interference), the sending station may resend the data frame.
The total elapsed time required for the data frame 20 and subsequent acknowledgment frame 22 to be transmitted in a SISO antenna configuration is shown in FIG. 2 as time TSISO. To a certain extent, the information contained in data frame 20 may be transmitted in less time using a multiple input, multiple output (MIMO) configuration such as that shown in FIG. 3. As shown, stations 10, 12 each includes a pair of antennas that communicate with the pair antennas on the other station. Thus, for example, antenna 40 can communicate with antenna 44 and antenna 42 can communicate with antenna 46, thereby establishing two simultaneously available communication links 48 and 50 between stations 10 and 12. This type of MIMO configuration is referred to as a “2×2” MIMO configuration, and other types of MIMO configurations exist in which more than two antennas at each station are implemented such as “3×3” MIMO, etc.
The advantage of a MIMO antenna configuration is illustrated with regard to FIGS. 4a-4c. FIG. 4a simply repeats the SISO frame exchange sequence from FIG. 2. As noted above, the time required to transfer the data and acknowledgment frames is TSISO. FIGS. 4b and 4c depict the frame exchange sequence using the 2×2 MIMO antenna configuration of FIG. 3. With MIMO, the bit stream can be broken into two parts and the parts can then be transmitted simultaneously via the two communication links 48 and 50. Thus, the overall time required to transfer the same information is advantageously reduced. In FIG. 4c, the total time is shown as TMIMO, which is less than TSISO. The time savings largely comes from being able to divide the data payload 28 of the data frame 24 into two smaller fields 52 and 54. Various techniques are known for doing this such as putting all of the even bits of data field 28 into field 52 and the odd bits into field 54. At the receiving station, the data parts 52 and 54 then can be reassembled into a single data payload.
Although the data field 28 advantageously can be broken apart for concurrent transmission, not all of the fields in the frames can be broken apart. Specifically, the preamble and header fields 24 and 26 must be maintained in their entirety. This is so because those fields contain information that is necessary for the proper reception of the data from the network. Also, the acknowledgment frame, being relatively small, is not broken apart. Thus, although 2×2 MIMO provides two independent and simultaneous communication links, communication throughput speed is not doubled.
The preceding discussion illustrates two problems for which solutions are highly desirable. One problem concerns how to take advantage of the increased communication speed provided by a MIMO antenna configuration. As noted above, a 2×2 MIMO configuration makes it possible to transmit twice as many bits in the same amount of time as in a SISO configuration. However, the overhead information, much of which cannot be broken apart, contained in typical wireless communication frames reduces the throughput gains that otherwise would be possible.
Another problem is that it is desirable to provide wireless networks that can be configured as flexibly as possible. For example, it might be desired for some stations to be SISO only while other stations are capable of MIMO communications. Further still, of the MIMO stations, it might be desirable for some stations to be configured as 2×2 MIMO, while other MIMO stations are 3×3 MIMO. It might also be desirable for some stations to reconfigure themselves for different types of MIMO or SISO configurations during operation as they communicate with other stations on the network. In general, MIMO stations may not know in advance which antenna configuration should be used to receive an incoming frame from the air.
Moreover, any improvement to the efficiency of wireless communications is desirable. A system that solves the problems described above and, in other respects, generally improves the efficiency of wireless channel utilization would be highly desirable.